This invention relates to plastic containers, such as beverage containers, that include a barrier coating to reduce gas permeation therethrough, wherein the barrier coating has enhanced resistance to loss of barrier properties caused by handling abuses and expansion of container walls.
Plastic containers comprise a large and growing segment of the food and beverage industry. Plastic containers offer a number of advantages over traditional metal and glass containers. They are lightweight, inexpensive, nonbreakable, transparent, and easily manufactured and handled. However, plastic containers have at least one significant drawback that has limited their universal acceptance, especially in the more demanding food applications. That drawback is that all plastic containers are more or less permeable to water, oxygen, carbon dioxide, and other gases and vapors. In a number of applications, the permeation rates of affordable plastics are great enough to significantly limit the shelf-life of the contained food or beverage, or prevent the use of plastic containers altogether.
It is known that a container structure that combines the best features of plastic containers and more traditional containers could be obtained by applying a glass-like or metal-like layer to a plastic container, and metallized plastic containers. For example, metallized potato chip bags have been commercially available for some time. However, in applications where the clarity of the package is of significant importance, metallized coatings are not acceptable. Obtaining durable glass-like coatings on plastic containers without changing the appearance of the container has proven to be much more difficult.
A number of processes have been developed to apply glass-like coatings onto plastic films, where the films subsequently are formed into flexible plastic containers. However, relatively few processes have been developed that allow the application of a glass-like coating onto a preformed, relatively rigid plastic container such as the polyethylene terephthalate (PET) bottles commonly used in the U.S. for carbonated beverages, and heretofore no process has been developed to provide application of a glass-like coating onto the external surface of a plastic container that is sufficiently durable to withstand the effect of pressurization of the container, that retains an enhanced barrier to gases and vapors subsequent to said pressurization, and that does not affect the recyclability of the containers. Pressurized beverage containers currently comprise a very large market world-wide, and currently affordable plastics have sufficiently high permeation rates to limit the use of plastic containers in a number of the markets served.
Such pressurized containers include plastic bottles for both carbonated and non-carbonated beverages. Plastic bottles have been constructed from various polymers, predominantly PET, particularly for carbonated beverages. All of these polymers, however, exhibit various degrees of permeability to gases and vapors, which have limited the shelf life of the beverages contained within them. For example, carbonated beverage bottles have a shelf-life which is limited by loss of CO2. (Shelf-life is typically defined as the time needed for a loss of seventeen percent of the initial carbonation of a beverage.) Because of the effect of surface to volume ratio, the rate of loss becomes greater as the size of the bottle is reduced. Small containers are needed for many market applications, and this severely limits the use of plastic bottles in such cases. Therefore, it is desirable to have a container with improved carbonation retention properties.
For non-carbonated beverages, similar limitations apply due to oxygen and/or water-vapor diffusion, again with increasing importance as the bottle size is reduced. Diffusion means both ingress and egress (diffusion and infusion) to and from the bottle or container. The degree of impermeability (described herein as xe2x80x9cgas barrierxe2x80x9d) to CO2 diffusion and to the diffusion of oxygen, water vapor, and other gases, grows in importance in conditions of high ambient temperature. An outer coating with high gas barrier can improve the quality of beverages packed in plastic bottles and increase the shelf life of such bottles, making small bottles a more feasible alternative, which presents many advantages in reduced distribution costs and a more flexible marketing mix.
It is also desirable that plastic containers such as PET bottles be recyclable. Known barrier enhanced coatings, however, are often organic and relatively thick and therefore can contaminate a recycled plastic product. Organic coating materials incorporated into recycled plastic make unsuitable containers for beverage or food items because the beverage or food items can contact the organic coating material and become contaminated. In addition, relatively thick coatings form relatively large particles during recycling of plastic material and can damage the appearance and properties of a resulting recycled plastic product. In particular, relatively large coating particles in recycled plastic can make otherwise clear plastic hazy. Hazy plastic is often undesirable for containers such as beverage and food containers.
Additionally, the cost of applying a coating to the outside of a bottle must not add significant cost to the basic package. This holds even when the coating is a gas barrier that significantly increases the shelf-life of beverage contained in that bottle, significantly reduces product spoilage of beverage contained in that bottle, significantly reduces product spoilage due to UV radiation, virtually eliminates environmental stress cracking, and/or provides a specific color. This criterion eliminates many processes for high gas barrier coatings, because plastic bottles are themselves a very low cost, mass produced article. Affordability implies in practice that the cost of the coating must add minimal or no increase to the cost of the whole package and in fact, the cost can be less.
A coating on the outside of plastic bottles must be capable of flexing. When bottles are used for pressurized containers, the coating preferably should be able to biaxially stretch whenever the plastic substrate stretches. It also is preferable that the coating be continuous over the majority of the container surface. Adhesion is particularly important in the case of carbonated beverages, since CO2 within the bottle exerts some or all of its in-bottle pressure on the coating. This pressure can rise to exceed 6 bar, exerting considerable forces on the coating/plastic interface. The coating must also resist scuffing, normal handling, weathering (e.g., exposure to rain, sun, and temperature fluctuations), and the coating must maintain its gas barrier throughout the bottle""s useful life.
There are several plasma-enhanced processes which apply an external, inorganic coating to a range of articles, which in some cases includes bottles. Many of the processes are targeted to provide coating properties which are quite different, and far less onerous than high gas barrier bottle coatings. Such processes target, for example, abrasion resistance, where the coating continuity is not a major factor, since the coating can protect the microscopic interstices. Other processes target cosmetic or light-reflection properties and some processes have a pure handling protection role. Often the substrate does not flex or stretch, and the article itself is higher priced than plastic bottles so that cost is not a benefit of the design. In some cases, the substrate allows far higher coating temperatures than those allowed by PET, the most common plastic-bottle material. Such processes generally do not provide the coating continuity, adhesion, and flexibility needed for high gas barrier coatings, nor do they provide a solution to the other problems associated with high gas barrier coatings described above.
PCT WO 98/40531, which addresses the foregoing deficiencies and problems, describes an electric arc process for the plasma-enhanced deposition of inorganic coatings onto a plastic substrate, under vacuum. A low voltage electric arc is generated between a cooled cathode and an anode, the anode being a crucible holding the coating solids, which are evaporated and plasma-energized by the energy of the arc. One or more reactive gases can be added to the plasma. A dense, well-adhering coating deposition is obtained due to a relatively large proportion of high-energy particles in the plasma. The method yields plastic containers having external glass-like barrier coatings.
Plastic containers in some applications, however, must withstand very high measures of handling abuse and/or conditions resulting in high expansion of the plastic substrate. High measures of handling abuse can occur in bottling plants, during distribution, and in the market. Examples of these abuses include (i) the use of empty bottle storage-silos and of bottle sorting/erection devices after these silos, which have the effect of continuously rubbing the bottles against each other and against metal contact parts, often severely scratching them; (ii) the use of bottle warmers that spray bottles with hot water, subjecting the surface to hot humid conditions and to the chemical effects of any water additives, such as de-scaling agents; (iii) the use of bottling lines designed to handle glass containers (not designed for the dedicated handling of PET containers), which lines generally are less gentle in handling plastic containers; and (iv) the distribution of containers over long distances in hot, dusty conditions, resulting in excessive damage due to container-to-container contact and abrasion. Conditions leading to high container expansion are usually the combination of high ambient temperature and humidity, particularly in the case of highly carbonated beverages (due to the high internal pressure of containers handling such beverages), and also particularly in the case of contour packages with decorative features (due to the weakening of the container wall by such features). Conditions of handling abuses and/or high container expansion, as given in the above examples, can lead to excessive coating damage, which will result in significant loss in barrier. These abuses and conditions can require coating design measures beyond those described in PCT WO 98/40531 in order to avoid a significant loss of barrier at the point-of-sale of some markets. A more robust coating would be advantageous in some applications.
It is therefore an object of the present invention to provide barrier coated plastic containers, and coating methods therefor, that reduce loss in barrier due to abrasion and other handling abuse and due to conditions leading to high expansion of plastic substrate.
It is another object of the present invention to provide an outer coating for a container, such as a heat sensitive plastic bottle, which has increased resistance to abrasion and other handling abuse and to conditions leading to high expansion of plastic substrate.
It is a further object of the present invention is to provide coatings for plastic containers having increased barrier as compared to that provided by the methods described in PCT WO 98/40531.
It is yet another object of the present invention to provide a coating and a system and method for coating which can provide an external glass-like coating that is flexible, durable and possess sufficient adhesion to withstand the effects of flexing, stretching, denting, and abrasion of the container, without significant loss of enhanced barrier properties.
Systems are provided for making a coated plastic container possessing a gas barrier and having enhanced resistance to loss in barrier due to handling abuses expansion of walls of the container. The system comprises (a) a vacuum cell capable of maintaining a vacuum within the vacuum cell; (b) at least one coating source disposed in the vacuum cell for supplying a coating vapor to an external surface of a plastic container positioned within the vacuum cell, wherein the coating source comprises an evaporator for heating and evaporating an inorganic coating material, such as metal or silicon, to form the coating vapor, and a means for energizing the coating vapor to form a plasma; and (c) gas feeds for supplying one or more process gases into an interior space of the vacuum cell, normally into the area of plasma-generation. At least one of the process gases is a carbon-containing gas, preferably a low-molecular weight organic gas, such as acetylene, ethylene, or ethane. The coating source is arranged within the vacuum cell such that the plasma reacts with at least one of the process gases and a thin coating is deposited and bonded on the external surface of the plastic container, such that the thin coating comprises carbon and inorganic material, the inorganic material being, for example, a clear/transparent inorganic oxide. The combination of coating components provides good gas barrier with enhanced resistance to loss in barrier.
Depending upon the selection of inorganic material and carbon-containing gas, a variety of barrier coatings can be produced, including colorless transparent oxide coatings; opaque or translucent coatings; and colored coatings.
The system can further include container feeders and conveyors for transporting multiple plastic containers into the vacuum cell and through coating process, preferably in a continuous or semi-continuous manner.
In another embodiment, the system comprises a vacuum cell and at least one main coating source as described above, along with (a) a gas feed for supplying one or more process gases into an interior space of the vacuum cell, and (b) at least one polymer coating source disposed in the same or another vacuum cell, for adding one or more coatings of polymer onto the plastic container positioned within the vacuum cell. The main coating source is arranged within the vacuum cell such that the plasma reacts with at least one of the process gases, which optionally includes a carbon-containing gas, and a thin coating is deposited and bonded on the external surface of the plastic container, such that the thin coating comprising inorganic material, such as an inorganic oxide. In a first variation of this system, the polymer coating source comprises a second gas feed comprising a polymerizable gas, and a means for energizing the polymerizable gas to form a plasma comprising polymerizable free radicals, such that the polymerizable free radicals deposit and polymerize to form a thin polymer coating on the plastic container. Examples of the polymerizable gas include olefins, paraffins, and mixtures thereof. Ethylene and acetylene are preferred. In a second variation of this system, the polymer coating source comprises a melter-evaporator for heating and evaporating a vaporizable polymer to form a polymer coating vapor, which recondenses and deposits to form a thin polymer coating on the plastic container. The vaporizable polymer is one that can be evaporable under vacuum conditions without decomposing, such as a polyolefin, a polyester, a polycarbonate, or a mixture thereof. Polyethylene is a preferred polymer. In either variation of the system, the thin polymer coating can be applied as a topcoat, an undercoat, or both, in relation to the main inorganic coating. The combination of inorganic coating, with or without optional carbon component, and polymer pre- and/or post-coat provides good gas barrier with enhanced resistance to loss in barrier.
In a preferred embodiment, the coated plastic container, when containing four volumes of carbon dioxide sealed in the interior volume, possesses a gas barrier of at least 1.25x the gas barrier of the plastic container without the coating, even after severe conditions of handling abuse and environments.
Methods for making coated plastic containers based on these systems are provided, along with the coated plastic containers themselves. Packaged beverages and packaging systems therefor also are provided.